Association between metabolic syndrome and hearing loss: The mediating role of retinol - A cross-sectional analysis of NHANES 2007 to 2018 (excluding 2013-2014).

Emerging evidence suggests associations between metabolic syndrome (MetS) and hearing loss, though population heterogeneity remains understudied. Given the high susceptibility of the inner ear to oxidative damage, fat-soluble antioxidants such as vitamins A, E, and carotenoids may exert protective effects. This study aimed to examine the MetS-hearing loss association among the U.S. adults and assess the potential mediating roles of these antioxidants. This cross-sectional study analyzed data from the U.S. National Health and Nutrition Examination Survey (2007-2018, excluding 2013-2014). Weighted multivariable linear regression models were employed to assess the association between MetS and pure-tone average (PTA) thresholds. Hearing loss was defined as a PTA ≥ 20 decibel in the better ear. Subgroup analyses explored effect modification by demographics (age, sex, race, education, marital status, income) and exposures (noise, ear infection history). Sensitivity analyses, including complete-case analysis and stratification by survey cycle, were conducted to test robustness. Mediation effect models were applied to evaluate the potential intermediary roles of vitamin A, vitamin E, and carotenoids. The study included 8759 U.S. adults (mean age 49.2 years; 51.1% female; 37.2% non-Hispanic White). Socioeconomically, 56.8% had college-level education or above, 59.2% were married or living with a partner, and 38.3% were in the middle-income tier. Significant noise exposure was reported by 48.9%, and 25.5% had a history of ear infections. MetS prevalence was 35.2%. After full adjustment, MetS showed a significant association with higher PTA thresholds after covariate adjustment (β = 1.292 decibel, 95% confidence interval: 0.315-4.106, P < .001). Subgroup analyses revealed stronger associations in older adults, non-Hispanic White, middle-income groups, and those with ear infection history (all interaction P < .05). Mediation analysis identified retinol (vitamin A) as the only significant mediator among 13 fat-soluble antioxidants, demonstrating an indirect effect of β = -0.358 (95% confidence interval: -0.847 to -0.061, P  = .020) with a mediation proportion of -13.3%. MetS is independently associated with hearing threshold elevation, with heterogeneity across subgroups. Retinol may play a crucial mediating role by modulating the inner ear oxidative-antioxidative balance. Prospective cohort studies and mechanistic investigations are needed to verify causality and elucidate underlying pathways, potentially informing novel strategies for preventing MetS-related hearing loss.